We are interested in studying asymmetric cell division (ACD), the process by which a mother cell divides to produce daughters that adopt different cell fates. Previous research in our lab identified
ham-1 as a regulator of ACD in the HSN/PHB neuroblast lineage. The wild-type HSN/PHB neuroblast divides to produce an anterior daughter that dies and a larger posterior daughter that survives and divides to produce HSN and PHB neurons.
ham-1 mutants have extra HSNs and PHBs because the anterior daughter of the HSN/PHB neuroblast inappropriately survives and is transformed into an HSN/PHB precursor. HAM-1 encodes a novel protein of 414 amino acids that is localized to the posterior of the HSN/PHB neuroblast and specifically inherited by the HSN/PHB precursor.Subsequent screens for mutants with the Ham-1 phenotype identified three recessive maternal-effect mutations in the
pig-1 gene (previously known as
bug-3). Several lines of evidence suggest that
pig-1 and
ham-1 act together to regulate ACD in this lineage. First,
ham-1 and
pig-1 mutants have extra HSNs and PHBs. Second, mutations in both
ham-1 and
pig-1 interact synergistically with mutations in the programmed cell death killer gene
ced-3 to regulate the penetrance of the extra neuron phenotype. These genetic interactions indicate that neither
ham-1 nor
pig-1 is a programmed cell death gene. Third,
pig-1 mutations are epistatic to
ham-1 mutations. In
ham-1 nulls, ~25% of the HSN/PHB lineages produce extra neurons and ~25% of these lineages are missing neurons.
pig-1 mutants have a much lower penetrance of extra neurons (~8%) and are not missing neurons. The double mutant looks like
pig-1. We propose two models to explain these results. In one model,
ham-1 and
pig-1 act in a linear pathway, and
ham-1 is a negative regulator of
pig-1. Alternatively,
ham-1 and
pig-1 act in parallel, and reducing the activity of
pig-1 alters the regulation of the lineage such that
ham-1 is no longer required.We cloned
pig-1 (
par-1-like gene) and found that it encodes a conserved member of the PAR-1 family of serine/threonine kinases. Experiments are underway to determine the PIG-1 expression pattern and the subcellular distribution of the PIG-1 protein. To test the hypothesis that
ham-1 is a negative regulator of
pig-1, we are performing PIG-1 overexpression experiments and examining the HSN/PHB lineage in
pig-1 mutants and
pig-1 ham-1 double mutants.